The embodiments herein relate to downhole electromagnetic telemetry systems and, more particularly, to insulative coating processes for electromagnetic telemetry antenna assemblies.
In measurement while drilling (MWD) applications, a variety of communication and transmission techniques are used to provide real time data from the vicinity of a drill bit to the surface during drilling operations. One technique uses a downhole antenna associated with the drill string and an MWD tool to transmit electromagnetic waves through the earth and to a receiver arranged at the surface. The receiver receives and records the electromagnetic data, thereby providing an operator with real time data associated with drilling parameters such as bit weight, torque, and wear and bearing conditions. MWD applications may also provide an operator with real time data associated with the physical properties of the subterranean formation being drilled such as pressure, temperature, and wellbore trajectory. Consideration of such information can result in faster penetration rates, better trip planning, reduced equipment failures, fewer delays for directional surveys, and the elimination of the need to interrupt drilling for abnormal pressure detection.
As an integral part of the MWD tool, the downhole antenna is housed in a mandrel that electrically isolates two portions of drill string, thereby creating suitable antenna capabilities. In order to electrically isolate the two portions of the drill string, the mandrel will typically include an insulative coating applied to its exterior surface. It has been found, however, that certain processes used in applying the insulative coating to the mandrel have resulted in coating inconsistencies and/or contamination. For instance, current coating processes often allow the coating to become contaminated by allowing moisture in the air or from cutting and sizing operations to permeate into the coating. As a result, the insulative coating will be more susceptible to failure in harsh downhole environments. Failure of the coating removes the electrical isolation, which equates to a failure of the antenna and the inability to perform MWD.